It is known in the art to use power operated heavy-duty chucks in machine tools of all kinds to clamp a workpiece to be machined. Such chucks are operated by hydraulically or pneumatically operated cylinder-and-piston means which are of known construction and connected to the clamping jaws of the chuck. The problem inherent during use of such power operated chucks consists in that it cannot be ascertained with sufficient certainty whether the workpiece to be machined is actually properly clamped after the chuck is operated. It could, for instance, happen that in view of diameter differences of various workpieces, the stroke of the piston in the clamping cylinder, and therewith that of the clamping jaws in clamping direction, reaches its end before the workpiece is properly clampled.
Furthermore, it could for instance happen, that after proper clamping of the workpiece, the conduit for feeding pressure fluid into the clamping cylinder becomes defective so that the workpiece would not be properly clamped any longer. In both cases the machine tool could start, respectively continue the operation with a non-properly clamped workpiece. In such a case the workpiece could be thrown out of the machine tool, which could lead to injury of the operator or damage of the machine tool.
In order to overcome these shortcomings it has already been suggested in the German Pat. No. DDR-PS 39001 to prevent sudden pressure loss, for instance due to defective pressure fluid conduits, by arranging in close vicinity to the clamping cylinder a check valve which, during pressure loss in the fluid pressure conduit, closes the cylinder space which is acted upon by the pressure fluid. Such an arrangement can, however, not prevent that, due to leakage losses in the check valve or due to leakage losses in the seal of the piston, the clamping force will gradually diminish, whereby nearly the same disadvantages will occur as by a suddenly diminishing clamping force. In the mentioned patent it has also been suggested to monitor the limits of the effective clamping region of the clamping cylinder by two limit switches. Clear statements as to the function of the two limit switches are, however, not contained in the aforementioned patent. From the illustration in FIG. 1 of this patent can, however, be visualized that one of the limit switches evidently signals the end position of the piston in a tandem cylinder, whereas the other limit switch signals the other end position, apparently in order to assure that the piston in the case of internal or external clamping has not moved to its end position without clamping the workpiece. However, these two limit switches do not afford proper clamping control since, for instance at defective seals of the piston, the clamping jaws may properly abut the workpiece, while the latter is not clamped with the necessary force. Likewise slow pressure drop and corresponding drop of the clamping pressure remains undetected so that the suggested check valve and limit switches in this known construction in their disclosed combination will not assure proper clamping of the workpiece during the machining operation.
The construction disclosed in the aforementioned German patent is modified in the No. DT-AS 1911558 solely in order to obtain shorter clamping cylinders. In this construction, it is likewise suggested to prevent certain pressure loss by the provision of check valves, to transfer the movement of the piston in the clamping cylinder to the exterior of the latter so as to control the piston position outside of the cylinder. From the mentioned DT-AS as well as from the mentioned DDR-PS can, however, not be ascertained in which way the knowledge of the piston position can contribute to an improvement of the clamping control.
Therefore, the above-mentioned shortcomings are inherent in the constructions disclosed in the aforementioned publications.